Displacement cascade

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

Edispiacement) from lattice sites. A sequence of elastic interactions results in a branching cascade of displaced atoms (Kinchin and Pease 1955), the displacement cascade. The dimensions of the cascade depend not only on Ej but also on the energy and mass of the incident particle. For a heavy ion the energy dissipation is high (>1 eV/atom), and the assumption of purely binary interactions breaks... 

The direct impact or cascade quenching model assumes that an amorphous domain forms directly within the core of a displacement cascade in a manner similar to the result of rapid quenching of a melt. Complete amorphization of a bulk specimen is achieved when the amorphous domains increase in abundance with increasing ion dose, eventually occupying the entire sample volume. In contrast, the point defect accumulation model assumes that the incoming particle produces isolated point defects, and amorphization occurs spontaneously when the local defect concentration reaches a critical value. A... 

Robinson, M.T. The influence of the scattering law on the radiation damage displace cascade. Phil. Mag. 12,741 (1965)... 

Transformation from the crystalline to amorphous state can be initiated by either homogeneous point defect accumulation or heterogeneous displacement cascade collisions (Trinkaus 1997). The prime requisite for this is the high mobility of radiation defects. However, in the case of oxide ceramics, the mobility of the radiation defect is very low so that thermally activated recrystallization occurs at high rate compared to the defect production (Motta 1990, Weber et al. 1994). Hence, all of the specimens remained crystalline for the investigated irradiation conditions as confirmed from the XRD studies except for the Li implanted MgO films. 

The papers [79,80] give consideration to the radiation tolerance of ferrite steel hardened with yttrium oxide (YjOj) particles of 20 nm. Two types of radiation treatment were used, in particular irradiation with helium ions that results in the formation of barely point defects and bombardment with high-energy (150 keV) iron ions. In the latter case, defects were mainly formed in displacement cascades, whose size ranges within 2-3 nm. It has been established that the changes in the distribution of dispersed phase and its component composition were not observed whatever methods were used to create defects. 

Pareige, R, Miller, M., Stoller, R. et al. 2007. Stability of nanometer-sized oxide elnsters in meehan-ically-alloyed steel under ion-induced displacement cascade damage eonditions. J. Nucl. Mater. 360 ... 

W J Phythian, R E Stoller, A E J Foreman, A F Calder and D J Bacon, A comparison of displacement cascades in copper and iron by molecular dynamics and its application to microstructural evolution, J. Nucl. Mater, 1995, 223, 245-261. 

Figure 4.15 Schematic representation of a a displacement cascade, and b a depleted zone open circles are vacancies filled circles are interstitial atoms) [40]...

Figure 6.20. Schematic of the sputtering process in SIMS. The incident ion buries itself and causes a displacement cascade in the material, while the ions leaving the sample come from the surface and are not much disturbed. (Adapted from Cameca [352] used with permission.)...

The displacement process produces two types of crystalline point defects, vacant crystaUine positions (vacancies) and displaced atoms in interstitial crystalline positions (interstitials). Vacancy and interstitial clusters are also created directly from displacement cascades. Molecular dynamics (MD) simulations show that the fraction of point defects surviving after the displacement cascade completely cools down is eventually only 20—40% of that predicted by the Norgett Robinson and Torrens (NRT) model [4]... 

D.J. Bacon, Y.N. Osetsky, R.E. Stoller, R.E. Voskoboinikov, MD description of damage production in displacement cascades in copper and alpha-iron, J. Nucl. Mater. 323 (2003) 152-162. 

Molecular-dynamics calculations provide valuable insight into the evolution with time of defect structures created in the collision caiscade. Consider, for example, the molecular-dynamics simulations of low-energy displacement cascades in the Bll-ordered compound CuTi (Figure 7) by Zhu et al, (1992). Figure 8 shows the number of Frenkel pairs produced by a Cu primary knock-on atom (PKA) as a function of recoil energy at the end of the collisional phase (0.2 p ) and at the end of the cooling phase (2.5 ps). The number of Frenkd... 

J. M. Delaye and D. Ghaleb. Molecular dynamics simulation of low energy atomic displacements cascades in a simplified nuclear glass. J. Nucl. Mater. 244,1997, 22. 

An important intermediate range of energies is from the so-called displacement cascade threshold to the sputtering threshold. Displacement cascades occur when a surface atom strikes an atom below, which strikes one below that, etc. until one atom ends up on an interstitial site. This sequence is shown in Figure 11.32. A displacement cascade is the lowest-energy process that results in a real defect in the sohd. Interstitial defects move rapidly. As long as they are close enough to the... 

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